The etching of painted surfaces that are exposed to environmental pollutants is a known problem. Acid rain is one of the main causes of this etching damage and can be seen as a gradual decrease in the gloss of the painted surface. Dark colored paints will often show this damage more readily than do lighter colored paints. In some cases, the damage can cause a surface to appear dull and hazy. To repair the damage, the surface must be buffed to remove the etched portions or the surface may need to be repainted.
The automotive paint industry is particularly concerned with this type of damage. Most automotive manufacturers have developed detailed procedures for determining the acid etch resistance of a paint. Before the automobile manufacturer will qualify a paint for use on its vehicles, it must pass a minimum level of acid etch resistance.
Today, most vehicles are coated with a paint system that comprises several layers of paint including a basecoat, which provides the color range of a painted vehicle, and a clearcoat that generally provides a high level of gloss and aesthetic value to the vehicles appearance. Such basecoat/clearcoat paints are applied in multiple layers. In a typical procedure, one or more layers of a basecoat composition are applied and subjected to a short drying period to remove at least a portion of the liquid carrier. One or more layers of a clearcoat composition can then be applied over the basecoat followed by a period of heat to dry and cure the applied layers of basecoat and clearcoat compositions.
The clearcoat composition is generally the outermost layers and is the most likely of the paint layers to be damaged by the effects of acid rain damage. Paint manufacturers have developed several types of paint formulations that are able to provide some resistance to acid rain damage. One commonly used paint formulation is that of an epoxy/acid clearcoat. In an epoxy acid clearcoat, the crosslinked network is formed by the reaction of epoxy groups with acid functional groups to form polyesters. One other commonly used clearcoat formulation is a silane clearcoat which adds silane functional groups to the crosslinkable and/or crosslinking components. Both of these approaches provide a higher level of resistance to acid rain than was available before their introduction. However, over time both of these types of clearcoats will show the effect of the action of acid rain.
It would be desirable for a clearcoat that has even higher levels of acid rain etch resistance than is currently available.